Cardiovascular disease is a major health risk throughout the industrialized world. Atherosclerosis, the most prevalent of cardiovascular diseases, is the principal cause of heart attack, stroke, and gangrene of the extremities, and thereby the principle cause of death in the United States. Atherosclerosis is a complex disease involving many cell types and molecular factors (for a detailed review, see Ross, 1993, Nature 362: 801-809). The process, in normal circumstances a protective response to insults to the endothelium and smooth muscle cells (SMCs) of the wall of the artery, consists of the formation of fibrofatty and fibrous lesions or plaques, preceded and accompanied by inflammation. The advanced lesions of atherosclerosis may occlude the artery concerned, and result from an excessive inflammatory-fibroproliferative response to numerous different forms of insult. For example, shear stresses are thought to be responsible for the frequent occurrence of atherosclerotic plaques in regions of the circulatory system where turbulent blood flow occurs, such as branch points and irregular structures.
The first observable event in the formation of an atherosclerotic plaque occurs when blood-borne monocytes adhere to the vascular endothelial layer and transmigrate through to the sub-endothelial space. Adjacent endothelial cells at the same time produce oxidized low density lipoprotein (LDL). These oxidized LDL's are then taken up in large amounts by the monocytes through scavenger receptors expressed on their surfaces. In contrast to the regulated pathway by which native LDL (nLDL) is taken up by nLDL specific receptors, the scavenger pathway of uptake is not regulated by the monocytes.
These lipid-filled monocytes are called foam cells, and are the major constituent of the fatty streak. Interactions between foam cells and the endothelial and SMCs which surround them lead to a state of chronic local inflammation which can eventually lead to smooth muscle cell proliferation and migration, and the formation of a fibrous plaque. Such plaques occlude the blood vessel concerned and thus restrict the flow of blood, resulting in ischemia.
Ischemia is a condition characterized by a lack of oxygen supply in tissues of organs due to inadequate perfusion. Such inadequate perfusion can have number of natural causes, including atherosclerotic or restenotic lesions, anemia, or stroke, to name a few. Many medical interventions, such as the interruption of the flow of blood during bypass surgery, for example, also lead to ischemia. In addition to sometimes being caused by diseased cardiovascular tissue, ischemia may sometimes affect cardiovascular tissue, such as in ischemic heart disease. Ischemia may occur in any organ, however, that is suffering a lack of oxygen supply.
The most common cause of ischemia in the heart is atherosclerotic disease of epicardial coronary arteries. By reducing the lumen of these vessels, atherosclerosis causes an absolute decrease in myocardial perfusion in the basal state or limits appropriate increases in perfusion when the demand for flow is augmented. Coronary blood flow can also be limited by arterial thrombi, spasm, and, rarely, coronary emboli, as well as by ostial narrowing due to luetic aortitis. Congenital abnormalities, such as anomalous origin of the left anterior descending coronary artery from the pulmonary artery, may cause myocardial ischemia and infarction in infancy, but this cause is very rare in adults. Myocardial ischemia can also occur if myocardial oxygen demands are abnormally increased, as in severe ventricular hypertrophy due to hypertension or aortic stenosis. The latter can be present with angina that is indistinguishable from that caused by coronary atherosclerosis. A reduction in the oxygen-carrying capacity of the blood, as in extremely severe anemia or in the presence of carboxy-hemoglobin, is a rare cause of myocardial ischemia. Not infrequently, two or more causes of ischemia will coexist, such as an increase in oxygen demand due to left ventricular hypertrophy and a reduction in oxygen supply secondary to coronary atherosclerosis.
The principal surgical approaches to the treatment of ischemic atherosclerosis are bypass grafting, endarterectomy, and percutaneous translumenal angioplasty (PCTA). The failure rate after these approaches due to restenosis, in which the occlusions recur and often become even worse, is extraordinarily high (30-50%). It appears that much of the restenosis is due to further inflammation, smooth muscle accumulation, and thrombosis.
Very recently, a modified balloon angioplasty approach was used to treat arterial restenosis in pigs by gene therapy (Ohno et al., 1994, Science 265: 781-784). A specialized catheter was used to introduce a recombinant adenovirus carrying the gene encoding thymidine kinase (tk) into the cells at the site of arterial blockage. Subsequently, the pigs were treated with ganciclovir, a nucleoside analog which is converted by tk into a toxic form which kills cells when incorporated into DNA. Treated animals had a 50% to 90% reduction in arterial wall thickening without any observed local or systemic toxicities.
Because of the presumed role of the excessive inflammatory-fibroproliferative response in atherosclerosis and ischemia, a number of researchers have investigated, in the context of arterial injury, the expression of certain factors involved in inflammation, cell recruitment and proliferation. These factors include growth factors, cytokines, and other chemicals, including lipids involved in cell recruitment and migration, cell proliferation and the control of lipid and protein synthesis.
For example, the expression of PDGF (platelet derived growth factor) or its receptor was studied: in rats during repair of arterial injury (Majesky et al., 1990, J. Cell Biol. 111: 2149); in adherent cultures of human monocyte-derived macrophages treated with oxidized LDL (Malden et al., 1991, J. Biol. Chem. 266: 13901); and in bovine aortic endothelial cells subjected to fluid shear stress (Resnick et al., 1993, Proc. Natl. Acad. Sci. USA 90: 4591-4595). Expression of IGF-I (insulin-like growth factor-I) was studied after balloon deendothelialization of rat aorta (Cercek et al., 1990, Circulation Research 66: 1755-1760).
Other studies have focused on the expression of adhesion-molecules on the surface of activated endothelial cells which mediate monocyte adhesion. These adhesion molecules include intracellular adhesion molecule-1, ICAM-1 (Simmons et al., 1988, Nature, 331: 624-627), ELAM (Bevilacqua et al., 1989, Science 243: 1160-1165; Bevilacqua et al., 1991, Cell 67: 233), and vascular cell adhesion molecule, VCAM-1 (Osborn et al., 1989, Cell 59: 1203-1211); all of these surface molecules are induced transcriptionally in the presence of IL-1. Histological studies reveal that ICAM-1, ELAM and VCAM-1 are expressed on endothelial cells in areas of lesion formation in vivo (Cybulsky et al., 1991, Science 251: 788-791; 1991, Arterioscler. Thromb. 11: 1397a; Poston et al., 1992, Am. J. Pathol. 140: 665-673). VCAM-1 and ICAM-1 were shown to be induced in cultured rabbit arterial endothelium, as well as in cultured human iliac artery endothelial cells by lysophophatidylcholine, a major phospholipid component of atherogenic lipoproteins. (Kume et al., 1992, J. Clin. Invest. 90: 1138-1144). VCAM-I, ICAM-1, and class II major histocompatibility antigens were reported to be induced in response to injury to rabbit aorta (Tanaka, et al., 1993, Circulation 88: 1788-1803).
Recently, cytomegalovirus (CMV) has been implicated in restenosis as well as atherosclerosis in general (Speir, et al., 1994, Science 265: 391-394). It was observed that the CMV protein IE84 apparently predisposes smooth muscle cells to increased growth at the site of restenosis by combining with and inactivating p53 protein, which is known to suppress tumors in its active form.
The foregoing studies are aimed at defining the role of particular gene products presumed to be involved in the excessive inflammatory-fibroproliferative response leading to atherosclerotic plaque formation. However, such approaches cannot identify the full panoply of gene products that are involved in the disease process, much less identifying those which may serve as therapeutic targets for the diagnosis and treatment of various forms of cardiovascular disease.